Detonation meter



D. R. DE BOISBLANC April 7, 1953 DETONATION METER Filed Feb. 6, 1948 2SHEETS-SHEET 1 INVENTOR.

' D.R. DE BOISBLANC A TTORNEYS April 7, 1953 D. R. DE BolsBLANc2,633,738

DETONATION METER Filed Feb. e, 194e 2 SHEETS- SHEET 2 IN V EN TOR.

A 7'7" ORNE VS Patented Apr. 7, 1953 DETONATION DIETER Deslonde R. deBoisblanc, Bartlesville, Okla., assignor to Phillips Petroleum Company,a corporation of Delaware 'Application February 6, 1948, Serial No.6,752

2 Claims.

termined magnitude. The output of the threshold device consists ofvoltage `waves representative of detonations in the engine cylinder.These voltage waves are amplied and fed to a first pulse generatingcircuit which transforms each Wave into a first exponential pulse whichdecays exponentially from the peak value of the corresponding voltagewave, and thence to a second pulse generating circuit which transformsthe successive exponential pulses linto second exponential pulses whoserate of decay is relatively small compared to the first pulses. Theoutput of the second generator is'then integrated and fed to a vacuumtube voltmeter which indicates the average intensity of knocking over apreselected period. The pulse generating circuits are connected in anovel circuit and the vacuum tube voltmeter utilizes a balanced circuitso that meter readings of great accuracy and stability are obtainable.

.It is an object of the invention to provide a detonation meterincluding the combination of novel pulsing and integrating circuits witha pickup device and threshold to provide a more accurate indication ofdetonation intensity than has heretofore been possible.

It is a further object of the invention toprovide a balanced vacuum tubevoltmeter circuit cooperating with the aforesaid elements which issufficiently sensitive to respond to variations in detonation intensitycaused by a change of a fraction of an octane number in a fuel used inthe test engine.

It is a still further object of the invention vto provide a method andmeans for varying parameters in the detonation meter circuits to permitthe detection and measurement of small differences indetonationintensity.

It is astill further object to provide a detonation meter which willduplicate the rating of fuels by the ASTM method more reproducibly andwith greater sensitiveness than a bouncing 2 pin, even when operated byan unskilled operator.

It is still a further object of the invention to provide a detonationmeter utilizing standard circuit components which may be easily andeconomically manufactured.

Various other objects, advantages and features of the invention willbecome apparent from the following detailed description and disclosuretaken in conjunction with the accompanying drawings, in which:

Figure 1 is a block diagram of the detonation meter;

Figure 2 is a graph showing the waveform at various stages of thecircuit of Figure 1;

Figure 3 is a schematic circuit diagram of the pickup and filter shownby Figure 1;

Figure 4 is a schematic diagram of the threshold shown by Figure 1;

Figure 5 is a schematic diagram of the pulse generating circuits andintegrator shown by Figure 1; and,

Figure 6 is a schematic circuit diagram of the entire detonation meter.

Referring now to the drawings in detail, and particularly to Figures 1and 2, the novel detonation meter comprises a pickup I0 for convertingpressure variations in a cylinder into electric currents. Such pickupsare well known in the art and, hence, no detailed description thereof isbelieved necessary. Preferably, I utilize a magnetostriction type ofpickup such as that shown invEldredge Patent 2,269,760. In Figure 3, thepickup In is shown mounted on an engine cylinder II having a piston I2disposed therein which may be reciprocated by a piston rod I3.

In general, the output of the pickup I0 has the waveform shown at I5 inFigure 2. From this figure, it will be noted that the electrical currentcomprises a main pressure Wave I6 representative ofthe pressurevariations caused by normal combustion in the cylinder. The current alsoincludes voltage components I'I, I3 representing the opening and closingof the exhaust valves, components I9 representing the operation of theintake valve, and components 2| representing the ignition of the chargein the cylinder I I. The current further includes a voltage Wave orcomponent 22 representative of detonation or knocking in the cylinderII. It will be understood that, when the engine is operating normallywithout knocking, the fuel in the cylinder is ignited and the ignitionzone spreads uniformly through the cylinder, as indicated by the mainpressure wave I6. However, when knocking occurs, there'is a suddenexplosion or detonation 3 in the cylinder and this detonation producessudden pressure variations of considerable magnitude thereby producingvoltage variations in the pickup which are distributed over a widefrequency spectrum.

The output of the pickup l is ied to a iilter 30 whose frequency rangeis so selected as to attenuate the undesired voltage components l1, i8,l0, and 2| to a considerable extent while permitting the detonation waveto pass therethrough with little relative attenuation. In Figure 3, thisfilter is shown connected to conductors 3i and 32 leading from thepickupiii. The lil-ter includes a pair of inductances 33, 34 which areconnected in series between conductor 3l and an output terminal 35.grounded at 35 and connected to another output terminal 31. The inductor34 is shunted by.a condenser 33 and the respective terminals ofinductances 33 and 34 are connected to filter condensers 39, 40 and 4lwhich, in turn, are connected to grounded conductor 32.

Where it is desired to duplicate the performance in fuel rating of theASTM bouncing pin, the circuit constants of the inductances and filtercondensers are so chosen as to pass a band of frequencies below 4,000cycles per second and attenuate or substantially eliminate higherfrequencies. However, in some cases, a more favorable signal to noiseratio may be obtained-by adjusting the lter so as to pass other bands offrequencies. With certain types of engines, it has been foundadvantageous to utilize a sharply tuned band pass lter resonating at6,500 cycles per second while, with other engines, it may beadvantageous to utilize high pass iilters passing frequencies above8,000 or 12,000 cycles per second. In the present application, it isdesired that the performance or" the bouncing pin meter be duplicated sothat the lter 30 is constructed to pass frequencies below some arbitrarycut 01T frequency between 2,000 and 4,000 cycles per second.

The iiltered electrical current has the waveform shown at 43 in Figure2. It will be noted that the main voltage wave 44 is substantiallyunaffected by passage Vthrough the iilter and has substantially the sameshape as main voltage wave lli. However, the voltage components l1, I8,i9, and 2i are attenuated by the lter and appear, respectively, at 45,46, 41, and 48 in the filtered wave. The detonation wave 22 appears at49 as a peak or pip extending above main pressure wave lill. Thisresults from the fact the high frequency part of the detonation spectrumhas been eliminated by the filter, the resulting voltage wave being freefrom sinusoidal components and being additive with respect to mainvoltage wave 44.

If a high pass nlter were utilized, the main voltage wave 44 would notappear in the filter output and the detonation wave 49 would includedamped sinusoidal components of the frequencies passed by the filter.

The current from filter 30 is fed to an amplier 50 which increases theamplitude of the various voltage components shown at 43 in Figure 2 butdoes not change their waveform appreciably. From the amplifier 50, theltered amplified current is fed to a threshold device 60 whicheliminates all voltage components of less than a predeterminedamplitude. As shown by Figure 4, the threshold device includes a biasedelectron tube 6l to which current is fed from input terminals E2, 03 ofthe threshold device, these input terminals being connected to theoutput cirrThe conductor 32V isV cuit of the amplier 50. The inputterminals are shunted by a resistor 64 and input terminal 63 is groundedat t5. A bias voltage is impressed upon the cathode of electron tube 6Iby a network including a potentiometer 81 which is connected to thecathode of tube 5I and has its arm connected to a resistor 63, thelatter part being shunted by large iby-pass condenser 69.V .The networkfurther'includes-.a voltage regulator tube 12 having one terminalconnected to the potentiometer 61, the other terminal or tube 12 beinggrounded at 13. The potentiometer is also connected .through a resistor14 to a positive power supply terminal 15. In this manner, a very steadybias is applied to the cathode of tube 6l. The plate or the anode oftube 6i is connected by a lead 16 to input terminal 62 therebycompleting `the input circuit of the threshold device. The output of thethreshold device appears across terminals 11 and 10, terminal 11 beingconnected through a coupling condenser 19 to the cathode of the tube andoutput terminal 18 being connected to ground at 13.

When the filtered amplified current from the amplier stage 50 isimpressed upon input terminals 62 and t3, voltage components having asuflicient magnitude to overcome the bias impressed upon tube lrpasstherethrough and appear across the output'terminals 11 and 18. However,voltage components which are not of suincient magnitude to overcome thebias impressed on tube 6l do not pass therethrough and do not appear inthe output vof tube 6l. It will also be noted that the voltagecomponents passed by the threshold stage are unidirectional, for thetube 6| functions as a rectiiier in addition to its function as athreshold device.v The bias on the electron tube 6I. is so regulated ibychoice of the values oi the bias network components that only the peakdetonation components pass through the threshold device while the mainpressure wave and undesired components 45, 46, 41, and 48 are rejected.Thus, as diagrammatically shown by Figure 2, the threshold passes vonlywaves having an amplitude greater than that indicated by the dotted line83 so that only the portion of detonation wave 40 above this line ispassed through the threshold stage. The resulting voltage wave is shownat 84 and consists of the portion of the detonation peak or pip whichextends above the line 83. This peak has very narrow width at its baseof the order of .001 second.

The electron tube 6l in Figure 4 has been shown as a diode but it willbeunderstood that other tubes may be utilized. If, for example, a triodeis used, the incoming signal is impressed upon the control grid and theb-ias vis regulated in such fashion that only components of the desiredmagnitude arepassed through the tube. In this case, the threshold stagemay have an amplifying function in addition to its threshold andrectification functions. It is also within the scope of the invention touse pentodes or other multiple electron tubes in a manner which will befamiliar to those skilled in the art.

The output of the threshold device is fed to an amplier from terminals11 and 18, thereby increasing the amplitude of voltage wave 84 withoutchanging its waveform appreciably. The output of theamplier is fed to afirst pulse generating circuit |00. This circuit has Yinput terminalsIUI and 102, the latter terminalY being grounded at 103. The pulsegenerating unit comprises an energyV storage. device .and means forslowly discharging said energy storage device. In the present circuit,the energy storage device is a condenser |04 and the discharging meansis a resistor |05 shunted across said condenser. However, the energystorage device may be any other suitable energy storage component, asthose skilled in the art will understand. One terminal of theresistance-capacitance unit |04, |05 is connected to grounded inputterminal |02 while the other terminal of the unit is connected to thecathode of a triode |06, the control grid of which is connected by alead |01 to input terminal The triode |06, which may be one section of a6SL7 dual triode, is connected in a cathode follower circuit and,accordingly, its anode is connected directly to a positive power supplyterminal |08 while the output is taken from the cathode by a conductor|09, it being understood that the output of the pulse generating circuitappears between conductor |09 and ground.

When an amplified detonation peak 84 is impressed upon the terminals |0|and |02, current passes through the triode |06 thereby chargingcondenser |04. This condenser then discharges slowly through resistorproducing an exponential pulse, such as indicated at l in Figure 2. Thevalue of condenser |04 is made sufliciently small that it can be chargedby tube |06 in an interval of time which is small as compared with thelength of .voltage waves 84 so that the pulses have a very steep wavefront. The voltage of each pulse decays much more slowly than thevoltage of the corresponding wave 84. Accordingly, the voltage wave 84is transformed into a continuous pulse of exponentially decayingwaveform. The time constant of the resistance-capacitance unit |04, |05is so chosen that the pulses 1| resulting from successive detonationpeaks 84 are spaced and do not overlap. Suitable values for obtainingthis result are a value of .01 mfd. for the condenser |04 and 1 megohmfor the resistor |05. The amplitude of each individual pulse isdetermined by the amplitude of the corresponding peak 84 since theamplitude of this peak determines the initial charge impressed upon thecondenser. T-he cathode follower type of coupling has been foundextremely advantageous in this circuit since it acts as a low impedancesource to charge the condenser |04 when a pulse 84 is present on thegrid of tube |06, and an almost innite impedance when no pulse isimpressed on the grid and while condenser |04 is discharging.

The output of the flrst pulser |00 is fed to a second pulse generatingcircuit |20. AIn Figure 5, this connection is represented by theconnection of conductor |00 to the control grid of a triode |2|. Thispulsing circuit includes an energy storage device, such as a condenser|22, and means for discharging said energy storage device, such as aresistor |23 shunted across the condenser. Th-e resistance-capacitanceunit |22, |23 is connected between the cathode of triode |2| and aground connection |24. The triode |2| is also connected in a cathodefollower circuit and, accordingly, has its anode directly connected to apositive terminal |25 of the power' supply, the output from the secondpulse generating circuit appearing between ground and a conductor 26attached to the cathode of triode The resistance-capacitance unit |22,|23 has a substantially higher time -constant than the unit |04, |05 ofthe first pulse generating stage |00. Thus," suitable values for these.components are af'value of 1 mfd. for condenser |22 and a value of 1megohm for resistor |23. Consequently, in the circuit shown, the timeconstant of the unit |122, |23 is approximately 100 times as great asthe time constant of the unit |04 and |05.

As a result, each time a pulse is impressed upon the grid of triode |2|,a sustained pulse of much longer duration is produced by the pulsegenerating circuit |20. When a seri-es of pulses is fed to the circuit|20 responsive to successive detonations in the cylinder, the pulsesproduced by circuit |20 are of sufficiently long duration as to overlapthereby producing a Voltage wave |27 having a crest |28 of generallysawtoothed configuration. The peak magnitude |20 of each tooth isproportional to the magnitude of the corresponding pulse by which it isproduced and the amplitudes of the pulses in turn, are proportional tothe peak intensities of the respective detonations in the cylinder. Accordingly, each detonation produces a tooth |20 on the Wave crest |23which is proportional in amplitude to the peak intensity of thedetonation. Of course, if there is no detonation over a period ofseveral cycles, the exponentially declining value of the crest |28 mayreach the aero axis of the curve. The purpose of generating pulses is tosustain the peak values of waves 04 long enough for tube |2| to chargethe relatively large condenser |22 to a value substantially proportionalto the peak value of pulses and waves S4.

. In connection with the graphs of Figure 2, it is to be understood thatthe time axis is not uniform for the respective voltage waves shown bythe iigure due tc the unduly large space which would be required todepict these graphs in their proper scale. Rather, these gures aremerely intended to provide an indication of the waveform at the thevarious stages of the detonation meter.

The output of the second pulse generator |20 is fed to an integratingcircuit |30 which comprises a resistor |31 having one terminal thereofconnected to conductor |23 and having its other terminal connected to awire |32 leading to an output terminal |33. An integrating condenser |34is connected between wire |32 and a grounded conductor |35 which isconnected to the other output terminal |35. The condenser |34 may have avalue of l mfd. and the resistor 13| may have a value of 12 to 30megohms. This circuit integrates the wave |27 produced by the secondpulse generating circuit |20 and forms a substantially smooth voltagewave |31 representing the average value of the pulses |29 as well as theaverage Value of peak detonation intensity over a plurality of. enginecycles. This integrated voltage is fed from the terminals |33, |36 to avacuum tube voltmeter |40 of novel construction which provides anaccurate measure of the average detonation intensity.

, The detonation meter constructed in accordance with the foregoingprinciples is very accurate and gives extremely reliable readings whichmay readily be taken by an unskilled observer. The meter has been foundto be sufficiently sensitive to detect variations in knocking intensitycaused by a variation of a fraction of an octane number in the rating ofthe fuel supplied to the engine cylinder. The sensitiveness andaccurateness of the meter result, to a large extent, from theV novelcathode follower pulsing circuits in combination with the integrator andthe novel balancing circuits provided in the vacuum tube voltmeter whichwill now be described.

Referring to Figure 6, the novel balanced voltmeter circuit includes adual triode |4| consisting of two triode sections |42 and |43. Theintegrated voltage appearing across the terminals |33 and E36 is fed tothe grid of triode |42. The grid circuit of triode M3 includes a pair ofserially connected resistors |44 and |45 forming a voltage divider, oneterminal of this unit being connected to the control grid of triode M3,the other terminal of the unit being connected to a grounded conductor|46 and the junction between these resistors being connected to resistor'M by a resistance 41. The values of these resistors are so chosen as tobalance the load impressed upon the control grid of triode |42 by theintegrating circuit ii. The dual triode |4| also utilizes the cathodefollower principle and, hence, the anodes of the respective triodesections are directly connected to positive terminals |138, M9 of thepower supply unit. The respective cathodes of dual triode Ml areconnected to the control grids of the triode sections 50, |51 of a dualtriode |52. A pair of resistors |53, |513 of equal value are connectedbetween grounded conductor M6 and the respective cathodes of dual triodeIM and a pair ci equal resistors 55 and |55 are connected from therespective cathodes of dual triode |52 through a common resistor |51 tothe grounded conductor Hit.

The anodes of dual triode |52 are connected to a resistance networkhaving two branches, one of which includes two resistors |58, |50 ofequal value having a center tapped resistor |60 connected therebetweenwith its center tap connected to a positive terminal itil of the powersupply. The second branch of the anode network consists of two resistors|62, |53 connected in series between the anodes of dual triode |52. Ameter teli is shunted across the resistor |53 and gives a very accuratereading of the voltage impressed upon the vacuum tube voltmeter |40 bythe integrating circuit |30. I obtain very accurate and sensitive meterreadings by the use of the balanced dual triode circuits and the cathodefollower coupling utilized between the two stages of the vacuum tubevoltmeter.

in Figure 6, the circuits already described in connection with Figure 1are indicated by like reference characters and, hence, need not befurther described herein. It will be noted, however, that integratingresistor |3| has been replaced by two serially connected resistances|66, |61 and that these resistors may be shunted by a third resistor |68by means of a switch |69. This circuit is provided for varying theintegrating eiect of condenser lSi upon the pulses produced by thecircuit 20.

The ampliiier 55 is of conventional construction and may inciude atriode 50| having its control grid connected to input terminal 35 and toa grounded load resistor 502. The cathode of triode tti is connected toground through a cathodebiasing resistor 503 which is shunted by acondenser 5055. The anode of triode 50| extends through resistors 505,506 and 50i to a positive terminal 50S of the power supply and theseresistors cooperate with the respective grounded filter condensers 569and |0. The anode of tube Edi is coupled through a condenser 5|| to agrounded attenuator 5 2 having its adjustable tap connected to thecontrol grid of a triode 5 3. The cathode of triode 5|3 is connecteddirectly to ground while the anode is connected through resistors 5|5,EIB and 501 to terminal 508, a ground- 8, resistors 5|5 and 5I6. Theanode of triode 5|3 is also connected through a coupling condenser 520to output terminal 62 and output terminal 63 is grounded. Forconvenience, the two triodes 50| and 5|3 may be included in a singleenvelope, as shown.

The amplifier 9B is likewise of conventional construction and mayinclude a triode 99| having its control grid connected to input terminal'V1 and to a grounded load resistor 902. The cathode of triode isconnected to ground through a bias resistor 903 while its anode isconnected through resistors 901i and 965 to a positive terminal supplyv906, a grounded filter condenser 007 being connected between resistors04 and 005. The anode is also connected through a coupling condenser'908 to a grounded attenuator 909, the tap of which is attached to thecontrol grid of a triode SID. The cathode of triode `gli! is connecteddirectly to ground while the anode is connected through a resistor QH toa positive power supply terminal 912. The anode of triode 9|() is alsoconnected to output terminal Ii through a coupling condenser M2 and theother output terminal 62 is grounded, these terminals being shunted by aload resistor 's 3. If desired, a cathode ray tube may be connected atIt! and iii? for reading the voltage waves as they are produced by thethreshold amplifier. For convenience, triodes |66 and Qld are enclosedin a single envelope as are triodes 6i and $0 l. `In order to secureuniformity triode i2! may also be part of a dual triode, the othersection having its electrodes connected together' and grounded.

A power supply unit e is also shown in Figure 6 and is of conventionalconstruction. This unit may include a power transformer Q5| having aprimary winding 952 connected to a fuse 853 and a plug 95s for a l1()volt line. The transformer 55 has a center tapped winding 955 forsupplying current to the heaters of the electron tubes previouslydescribed and the center tap is grounded at 958. Transformer y95| alsoincludes a center tapped secondary winding e30 which is connected to theanodes of a dual rectifier tube 9M, the center tap being grounded at952. The transformer also includes a center tapped Winding 953 forsupplying lament current to rectiiier `.flll and this center tap isconnected through a choke 955 and a resistor SESS to a positive bus bar961. A ltering unit may be provided comprising condensers @'58 and556i?, regulator tubes 97B, 91| and bleeder resistors 972 and S73.Current from the bus bar 57 is supplied to the positive power supplyterminals 15, |48, |51, |49, |25, |08, `503, S, and 9|2.

The operation of the complete detonation meter will now be apparent tothose skilled in the art. When the engine is operated, the pickup I0generates voltages substantially proportional to the intensity ofdetonations in the cylinder together with voltages responsive to themain pressure wave and unwanted voltage components representing valveclatter and other disturbances in the cylinder. These voltages then passthrough the lter in which the constants of the capacitors andinductances are so chosen as to attenuate the unwanted componentswithout causing a serious decrease in the strength of the detonationvoltage wave. The filtered voltage is then amplifled and fed to thethreshold `|30 where all voltages below a predetermined amplitude areeliminated. The output of the threshold is a series ed filter condenserY5||i being connected between u of voltage Waves each having anamplitude pro- 9 portional to the peak intensity oi' correspondingthereto.

The amplitude of these voltage waves is then increased by amplier 90 andthe amplified waves are fed to the pulsing circuit wherein they aretransformed into spaced exponential pulses having amplitudesproportional to the respective peak detonation intensities. .Theseexponential pulses are fed to the second pulse generator |20 in whichthe spaced pulses are converted into overlapping ,pulses of longerduration, due to the substantially higher time constant of theresistance-capacitance unit in pulsing circuit |20. The resulting wavehas a crest of generally sawtoothed conguration, the peak of each toothhaving an amplitude proportional to the peakintensity of thecorresponding detonation. The output of the second pulse generator thengoes to the integrator 30 where a smooth steady voltage is producedwhich is proportional to the average peak detonation intensity indicatedby a series or plurality of voltage Waves produced by successivedetonations in the cylinder. This integrated voltage is read upon vacuumtube voltmeter |40 in which accurate readings are obtained due to thenovel balanced circuit utilized.

It is a feature of the invention that differences in detonationintensity caused by the use of reference fuels close in octane numbermay be spread over a large portion of the dial in voltmeter |64.Assuming that the internal combustion engine, not shown, is knockingevery cycle with the same intensity on a certain fuel such that meter|64 reads D1, then upon change to a fuel of slightly higher octane valueit will be found that the meter reads D2 which is less than D1. Denotingh1 and h2 as the average knock intensity at the pickup l0 which producedD1 and D2 respectively; then as:

b=amplication of 50 adjusted at 5 I2 c=threshold value of 60 adjusted atd=amplification of 90 adjusted at '909 then the detonation ha to innity.By varying the parameters b, c and d any desired ratio of D1 to D2 isobtained which permits detection and measurement of small differences inlarge quantities. This permits the use of reference fuels close inoctane number while providing proper spread on the dial of meter |164 asrequired by the ASTM test procedure for the bouncing pin meter.

An electronic instrument has been disclosed above which does not requirean expert operator, as is necessary with the ASTM bouncing pin. Nointricate mechanical adjustments are necessary in the present invention.More accurate and more sensitive results are obtained and yet theseresults have the same standard of rating as the bouncing pin vso thatresults may be compared directly with those made by the ASTM bouncingpin. In addition, the novel voltmeter and pulsing circuits contributeincreased accuracy which provides a more dependable reading than hasheretofore been obtainable,

While the invention has been described in con-y nection with a present,preferred embodiment,

thereof, it is to be understood that this description is illustrativeonly and is not intended to limit the invention, the scope of which isdened by the appended claims.

What is claimed as new and desired to be secured by Letters Patent is:

l. In a detonation meter, a pickup for converting pressure variations inan engine cylinder into electrical current comprising voltage componentsrepresentative of unwanted vibrations, voltage components representativeof the main pressure variations in the cylinder, and voltage wavesrepresentative of detonation, each voltage wave having an amplitudeproportional to the peak intensity of a detonation in the cylinder, alow-pass iilter for attenuating said unwanted voltage com- 1 ponents,the output of said lter including the vmain pressure waves with pipsprojecting therefrom representing the respective detonation voltagewaves. a threshold circuit for eliminating voltages of less than apredetermined amplitude which is slightly greater than the peakamplitude of the main pressure waves fed to the threshold whereby theoutput of said threshold consists of the detonation pips, a i-lrstpulsing circuit including a set of input terminals fed by said thresholdcircuit, a vacuum tube having an anode, a cathode and a control grid, aresistance connecting said cathode with one input terminal, a condenserconnected in parallel with said resistance to define aresistance-capacitance unit, a lead connecting said control grid to theother input terminal, means for supplying operating potentials to theelectrodes of said tube, a second pulsing circuit including a secondvacuum tube having an anode, a cathode and a control grid, a iixedresistance connecting the cathode of said second vacuum tube to ground,a condenser connected in parallel to said last-mentioned fixedresistance to denne a second resistance-capacitance unit, the timeconstant of said second unit being of the order of times the timeconstant of the iirst resistance-capacitance unit, a lead connecting thecontrol grid of said second tube to the cathode of said first tube, anintegrating circuit including a pair of output terminals, a xedresistance connecting the cathode of said second tube to one outputterminal, and an integrating condenser having its terminals connected tothe respective output terminals, the time constant of the integratingcircuit being several times greater than that of the secondresistance-capacitance unit, and means for measuring the voltageappearing across said output terminals.

2. In a detonation meter, a pickup for converting pressure variations inan engine cylinder into electrical current comprising voltage componentsrepresentative of unwanted vibrations and voltage waves representativeof detonation, each voltage wave having an amplitude proportional to thepeak intensity of a detonation in an engine cylinder, a filter forattenuating said unwanted voltage components, a threshold circuit foreliminating voltages of less than a predetermined amplitude whereby theoutput of said threshold consists of voltage waves of greater than saidpredetermined amplitude, a rst pulsing circuit including a set of inputterminals fed by said threshold circuit, a vacuum tube having an anode,a cathode and a control grid, a resistance connecting said cathode withone input terminal, a condenser connected in parallel with saidresistance to dene a resistance-capacitance unit, a

llk

lead connecting said control grid to the other input terminal, means forsupplying operating potentials to the electrodes of said tube, a secondpulsing circuit including a second vacuum tube having an anode, acathode and a control grid, a fixed resistance connecting the cathode ofsaid second Vacuum tube to ground, a condenser connected in parallel tosaid last-nentioned fixed resistance to define a secondresistance-capacitance unit, the time constant of said second unit beingof the order of 100 times the time constant of the firstresistance-capacitance unit, a lead connecting the control grid of saidsecond tube to the cathode of said rst tube, an integrating circuitincluding a pair of output terminals, a xed resistance connecting thecathode of said second tube to one output terminal, and an integratingcondenser having its terminals connected to the respective outputterminals, the time constant of the integrating circuit being severaltimes greater than that of the second resistance-capacitance unit, and avacuum tube voltmeter for measuring the voltage appearing across saidinput terminals.

DESLONDE R. DE BOISBLANC.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,251,973 Beale et al Aug. 12,1941 2,337,522 Eldredge Dec. 21, 1943 2,340,714 Traver et al. Feb. l,1944 2,378,846 Hansell June 19, 1945 2,416,614 Crossley et al. Feb. 25,1947 2,424,312 Haynes July 22, 1947 2,448,323 De Boisblanc Aug. 31, 1948OTHER REFERENCES Publication of Abstract of Serial 673,221 of DeBoisblanc, published January 31, 1950, vol. 630 of Oiiicial Gazette.

